/Engine/Plugins/Online/OnlineFramework/Source/Party/Public/Party/PartyTypes.h
Gives a smidge more meaning to the intended use for the string. These should just be UniqueId's (and are), but not reliably allocated as shared ptrs, so they cannot be replicated viaFUniqueNetIdRepl.
typedef [FString](API\Runtime\Core\Containers\FString) FSessionId